Presidents do still have the power, at least to a certain extent, to end all life on the planet with their actions. Yes, there are a lot of checks to make sure that's very hard. But it still almost happened several times by accident.
Also, Shigetsu could have created a system that allowed humans to use Magic in a way that simply didn't allow anybody to use magic to harm others. Hell, she could have even created a system of magic that would have the kind of checks and balances nukes have. She could have done anything, yet still chose to do something utterly stupid instead for petty and selfish reasons. >>195863269>That's a temporary fix, it wouldn't prevent awful things from happening in the future.
Like what? Doesn't she own all of the magic in the world after winning Granbelm?